(1) Field of the Invention
The present invention relates to a method and to a device for assisting low altitude piloting of an aircraft.
The technical field of the invention is the field of fabricating piloting assistance systems for use on board rotorcraft.
(2) Description of Related Art
Low altitude flight constitutes a difficult operation. In order to avoid obstacles, a pilot can rely only on seeing the surrounding space, on external aids such as sensors and cameras, usually without distance information, and on the pilot's knowledge of the aircraft being flown. The pilot must thus evaluate the presence of potentially dangerous obstacles on the path being followed and must also evaluate the ability of the aircraft to avoid those obstacles, given the aircraft's maneuverability.
Under conditions of poor visibility or at night, low altitude flights are that much more difficult to carry out. The pilot may have difficulty in seeing potential obstacles under poor environmental conditions.
Consequently, an aircraft may include a piloting assistance device providing information for indicating the presence of obstacles and enabling them to be avoided.
Patent FR 2 712 251 describes a method of assisting piloting in which a guard curve is used to determine the obstacles that are the most dangerous for the aircraft.
Such a guard curve may be constituted essentially by a circularly arcuate segment. The circularly arcuate segment presents a radius equal to the sum of the minimum pull-up and minimum pitch-down radii that are acceptable for the aircraft. Furthermore, the guard curve may include a horizontal segment upstream from the circularly arcuate segment. Under such circumstances, the guard curve has a horizontal portion extended by a circularly arcuate segment, thus presenting the shape of a ski presenting an upwardly curved tip.
The guard curve may be associated with a guard height, the guard curve being positioned at a height below the aircraft that is equal to the guard height.
Anything situated above the guard curve thus presents a potential obstacle.
In that method, a piloting assistance device divides the forward field situated in front of the aircraft into a plurality of angular sectors. The forward field represents the space that can be reached by the aircraft starting from its current position.
The piloting assistance device then acts for each sector to detect obstacles by means of a telemeter detector. The piloting assistance device then compares the tops of the obstacles with the guard curve.
In each angular sector, the piloting assistance device defines tops that are situated higher than the guard curve as being dangerous obstacles. For example, the altitude of each obstacle is calculated. Under such circumstances, the piloting assistance device determines a difference in altitude between the altitude of each obstacle and the point of the guard curve situated above or below the obstacle. The most dangerous obstacle is the obstacle for which the altitude difference presents the greatest algebraic value.
Each dangerous obstacle is shown to a pilot by being superposed on an image of the external landscape, which image also includes crosshairs representing the speed vector of the aircraft collimated at infinity. By way of example, dangerous obstacles are represented in the form of respective crosshairs. The piloting assistance device may also display a smoothed safety curve that is situated at a guard height above the dangerous obstacles. In order to fly as close as possible to the obstacle, the pilot must then cause the crosshairs representing the speed vector to lie on said curve.
Nevertheless, the presence of the crosshairs representing the speed vector at a location above the safety curve does not guarantee that the aircraft is in complete safety. For example, if the aircraft is flying in a valley, a turning maneuver might cause the aircraft to face a wall that could be difficult to avoid.
Furthermore, if at some instant the pilot does not comply with the procedure that should be applied, then a problematic situation can arise.
In such a situation, a first obstacle may then lie below the speed vector of the aircraft and at a first height above the guard curve.
A second obstacle may lie above the speed vector and at a second height above the guard curve. If the second height is less than the first height, then the first obstacle is designated as being the most dangerous obstacle. The pilot may be unworried since the obstacle designated as the most dangerous obstacle is situated below the speed vector. Nevertheless, the second obstacle represents a potential danger.
Patent FR 2 886 439 describes an alternative guard curve.
That guard curve has a first circular arc presenting a radius equal to the sum of the minimum pull-up radius authorized for the aircraft plus a ground guard height.
Furthermore, the static guard curve includes a second circular arc downstream from the first circular arc. The second circular arc presents a radius equal to the sum of said minimum pull-up radius authorized for the aircraft plus a minimum pitch-down radius authorized for the aircraft. The first circular arc and the second circular arc present a common tangent at the point where they join together.
The first circular arc then extends from a point situated on a straight line passing through the aircraft and through the center of the circle containing said first circular arc to the second circular arc.
Furthermore, a distance D between the guard curve and the obstacle is calculated. A pitch-down or pull-up order
      d    ⁢                  ⁢    φ        d    ⁢                  ⁢    t  is determined in application of the following formula:
            d      ⁢                          ⁢      φ              d      ⁢                          ⁢      t        =      G    *          (              D        +                  τ          *                                    d              ⁢                                                          ⁢              D                                      d              ⁢                                                          ⁢              t                                          )      where “G” is a gain and “τ” is a warning time.
Under such circumstances, a pull-up order is given when the sum
  (      D    +          τ      *                        d          ⁢                                          ⁢          D                          d          ⁢                                          ⁢          t                      )
is negative. A pitch-down order is given when the sum is positive.
Documents US 2011/210871, US 2010/042273, US 2007/265776, and US 2008/208400 are also known.